1. Field of the Invention
The present invention relates to a process for producing microcapsules whose walls are fabricated from a polyvinyl alcohol polymer.
2. Description of the Prior Art
Processes are known in which the walls of microcapsules are prepared by liquid-liquid phase separation using polyvinyl alcohol type polymers in an aqueous medium (French Pat. No. 1,304,891; Examined Japanese Patent Publication No. 72-26575; U.S. Pat. No. 3,574,133 and British Pat. No. 1,198,412).
In these processes, however, the polyvinyl alcohol polymers used are soluble in water at temperatures within the range from 0.degree. C. to 100.degree. C. and therefore the liquid-liquid phase separation of a simple aqueous polyvinyl alcohol solution cannot be induced without the aid of a third substance. Therefore, it is essential in the prior art processes to add a third substance capable of inducing the phase separation of the aqueous polyvinyl alcohol polymer solution thereby causing liquid-liquid phase separation. Suitable such phase separation inducers include inorganic salts such as sodium sulfate; organic solvents which are soluble in water but which behave as nonsolvents for polyvinyl alcohol such as isopropyl alcohol, and macromolecules soluble in water but immiscible with polyvinyl alcohol such as gum arabic. The phase separation inducers cannot be materials which are themselves wall formation components but are instead impurities in the polyvinyl alcohol polymer which becomes the wall material. It is necessary therefore to remove the phase separation inducer from the membrane by washing or some other treatment after microcapsule formation. Such a post wall formation treatment is disadvantageously complicated and troublesome.
The above conventional processes also have the disadvantage that since the phase separation inducer which is incorporated in the capsule wall membrane is removed during the step in which the impurities are removed, the microcapsules obtained are apt to be porous thereby possibly allowing the material in the interior of the microcapsules to escape. Moreover, another problem with the conventional processes is that it is difficult to adjust the amount of and the rate of addition of the phase separation inducer. If the rate of addition is too fast or the amount is excessive, the capsules are apt to congregate into group-like bunches. Conversely, if the rate of addition is too slow or the amount added is insufficient, a fairly prolonged period of time is required for microcapsulation. A need therefore continues to exist for an improved microcapsule formation process which overcomes the problems inherent in the use of a phase separation inducer.